Ignore case with string.Contains() in C#

To make it case-insensitive, you can use StringComparison.OrdinalIgnoreCase.

For example:

string sentence = "The quick Brown fox jumps over the lazy Dog.";

if (sentence.Contains("brown", StringComparison.OrdinalIgnoreCase))
{
    Console.WriteLine($"'{sentence}' contains 'brown'");
}

Output:

'The quick Brown fox jumps over the lazy Dog.' contains 'brown'

This works because it's performing a case-insensitive search, and it matches "brown" with "Brown" in the sentence.

Use string.IndexOf() with StringComparison

If you are working with an older version of .NET that doesn't support the string.Contains() overload with StringComparison, you can use string.IndexOf() instead, which works similarly.

string sentence = "The quick Brown fox jumps over the lazy Dog.";

int index = sentence.IndexOf("brown", StringComparison.OrdinalIgnoreCase);

if (index > -1)
{
    Console.WriteLine($"'{sentence}' contains 'brown', starting at index {index}");
}

Output:

'The quick Brown fox jumps over the lazy Dog.' contains 'brown', starting at index 10

In this case, it finds "brown" starting at index 10 in the string. This is how string.Contains() works internally, but with IndexOf(), you also get the starting position of the substring.